librgw: incremental enumeration progress

librgw: incremental enumeration progress

As of this change, enumeration is -almost- correct, but the
rules for name, object_name(), and full_object_name() need to be
checked.

The rationale for storing just the last component name in dirents
means that name can't ever have the form "foo/bar" (make sure it
doesn't).  But there may still be code which assumes name ==
object_name() OR assumes object_name() is hashable (it isn't, that
has to be the full object name).

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>